1. Field of the Invention
The present invention relates to a structure of a bonding apparatus and to a bonding method used for the bonding apparatus.
2. Description of Related Art
Wire-bonding apparatuses that connect between a pad as an electrode of a semiconductor chip and a lead as an electrode of a lead frame a wire as a thin metal wire are widely used in manufacturing semiconductor devices. In many cases, such wire-bonding apparatuses employ a method of sequentially bonding between a pad and a lead with a wire by inserting the wire through a capillary, forming a tip end of the wire that extends out from the tip of the capillary into an initial ball by, for example, sparking, moving the capillary down toward the pad, forming a pressure-bonded ball by pressure-bonding the initial ball to the pad using the capillary tip, moving the capillary upward, looping the wire from the pressure-bonded ball toward the lead while feeding out the wire from the capillary tip, moving the capillary down toward the lead, pressure-bonding the wire to the lead using the capillary tip, cutting the wire by moving the capillary upward, and then moving the capillary to a position of a pad to be processed next.
When carrying out wire-bonding in this manner, it is required to adjust the position of the capillary tip with the position of a pad or a lead to be bonded. According to a positioning method that is commonly used, the position of a pad or a lead is obtained by taking an image of a semiconductor chip or a lead frame using, for example, a camera, and measuring the distance by counting the number of pixels in the image between the position of the pad or the lead and the optical axis of the camera located at the center of the field of view.
When using this method, if the optical axis of the camera is disposed coaxially with the central axis of the capillary, the distance between the optical axis of the camera and the pad or the lead directly corresponds to the distance between the central axis of the capillary and the pad or the lead. Therefore, it is possible to carry out bonding with the position of the capillary tip being adjusted with the position of the pad or the lead by moving the capillary tip only by this distance. However, providing the optical axis of the camera coaxially with the central axis of the capillary causes the field of view to be blocked with the capillary and a bonding arm, and thus it is not possible to focus on the pad or the lead to be bonded. Therefore, the optical axis of the camera is provided at a position offset from the central axis of the capillary. Accordingly, positioning the capillary tip at the position of the pad or the lead based on the image acquired with the camera requires movement of the capillary tip by a distance including the distance between the optical axis of the camera and the pad or the lead as well as an offset amount.
A wire-bonding apparatus typically includes an X motor and a Y motor provided for an X-Y table and driving a bonding head, to which, for example, a capillary and a camera are attached respectively, in the X direction and the Y direction, and further includes a Z motor for driving a bonding arm to which the capillary is attached at its tip in the Z direction, and a source of heat generation such as a heat block for heating the lead frame. Accordingly, a temperature change of the wire-bonding apparatus resulting from the source of heat generation can often cause variations in relative positions between the central axis of the capillary and the optical axis of the camera, and error due to such variations appears as a displacement of the bonding position, or a displacement of the position of a pressure-bonded ball with respect to the pad.
Japanese Unexamined Patent Application Publication No. H07-297220, for example, proposes a method of moving the optical axis of a camera above the central position of a pressure-bonded ball that has been bonded, and correcting an offset amount based on the travel distance and the offset at this time. Further, Japanese Unexamined Patent Application Publication No. H08-31863, for example, proposes a method of, when bonding, detecting edges of a pressure-bonded ball based on three directions of lines respectively connecting the center of a pad that has been previously known and indicates the portion for wire binding and points within the pad and outside the pressure-bonded ball, calculating the central position of the pressure-bonded ball based on the three edges of the pressure-bonded ball, and correcting an offset amount based on the amount of displacement between the central position of the pad and the central position of the pressure-bonded ball. Japanese Unexamined Patent Application Publication No. H08-31863 alternatively proposes a method, as a method of detecting the central coordinate of a pressure-bonded ball, of setting an assumed ball center, detecting edges of the pressure-bonded ball for total eight directions including an X direction, a Y direction, and directions tilted 45 degrees respectively from the X direction and the Y direction in both negative and positive sides taking the assumed ball center as a reference point, dividing the directions into four groups and drawing perpendicular lines from respective central coordinates, and acquiring the central position of the pressure-bonded ball based on a coordinate of an intersection point of these lines.
Further, the size of pressure-bonded ball changes depending on, for example, the heating temperature of the heat block. A pressure-bonded ball smaller than a designed value can often result in poor bonding, and a pressure-bonded ball larger than the designed value can often cause the pressure-bonded ball to protrude from the pad. Accordingly, during bonding, it is required to detect the size of the pressure-bonded ball and confirm that the pressure-bonded ball is bonded at a designed size. Therefore, Japanese Unexamined Patent Application Publication No. H08-31863 proposes a method of acquiring the size of a pressure-bonded ball in the same manner when the central position of the pressure-bonded ball is acquired.
As the size of pads has increasingly become smaller along with fine-pitching of devices in recent years, there are manufactured semiconductor devices in which the diameter of a wire that extends from the pad is substantially the same as that of a pressure-bonded ball. When acquiring the image, using a camera, of a pressure-bonded ball that has been pressure-bonded to a pad of such a semiconductor chip, almost a half of the circumference of the outer shape of the pressure-bonded ball is hidden under the wire that extends from the pressure-bonded ball, and it is not possible to detect its edge by image processing. According to the alternative method disclosed in Japanese Unexamined Patent Application Publication No. H08-31863, it is difficult to acquire the central position of the pressure-bonded ball when there is more than one edge of the pressure-bonded ball that cannot be detected as being covered under the wire. Likewise, it is also difficult to acquire the size of the pressure-bonded ball.
According to the method of calculating the central position and the size of the pressure-bonded ball based on the central position of the pad and the three edges of the pressure-bonded ball as proposed by Japanese Unexamined Patent Application Publication No. H08-31863, it is often not possible to detect the coordinates of the edges as the intersection points respectively of an X axis and a Y axis that pass the center of the pad and the edges of the pressure-bonded ball are covered with the wire depending on the position of the pad and the position of the pressure-bonded ball. Therefore, the methods of correcting the offset amount and detecting the size of the pressure-bonded ball respectively disclosed by Japanese Unexamined Patent Application Publication No. H07-297220 and Japanese Unexamined Patent Application Publication No. H08-31863 cannot be applied to bonding for fine-pitch semiconductor chips in which the diameter of the pressure-bonded ball is substantially the same as that of the wire.